The present invention relates to a bandgap voltage reference circuit for producing a stable TlnT temperature curvature corrected voltage reference, which preferably is suitable for fabrication in a CMOS process, and the invention also relates to a PTAT voltage generating circuit for generating a PTAT voltage with a temperature curvature complementary to an uncorrected TlnT temperature curvature CTAT voltage of the type developed across a base-emitter of a transistor, which preferably is suitable for fabrication in a CMOS process. The invention also relates to a method for producing such a voltage reference and a PTAT voltage.
Most electronic circuits require a stable DC voltage reference, and in particular, a temperature stable DC voltage reference. Bandgap voltage reference circuits for producing a reasonably temperature stable DC voltage reference are known. Such bandgap voltage reference circuits rely on the property of a bipolar transistor to produce a substantially constant base-emitter voltage, and when fabricated in silicon, rely on the property of silicon which when a bipolar transistor is fabricated in silicon produces a base-emitter voltage in the range of 0.5 volts to 0.8 volts. However, the voltage produced by the base-emitter of a transistor has a negative temperature coefficient, in other words, the voltage is complementary to absolute temperature (CTAT). In known bandgap voltage reference circuits a pair of transistors are operated at different current densities and are arranged to develop a voltage which is proportional to the difference in the base-emitter voltages of the two transistors. This difference voltage has a positive temperature coefficient, in other words, the voltage is proportional to absolute temperature (PTAT). The PTAT voltage provided by the difference in the base-emitter voltages is properly scaled and summed with the CTAT voltage of one of the transistors to produce the voltage reference. However, as well as the linear relationship with temperature of the CTAT base-emitter voltage of a transistor, the CTAT base-emitter voltage also exhibits a non-linear temperature relationship which is referred to as temperature curvature. This non-linear relationship of the CTAT voltage to temperature is commonly represented by the term K.TlnT where K is a constant and T is absolute temperature in degrees Kelvin (xc2x0 K). Thus, in order to produce a voltage reference which is entirely temperature stable over a reasonable temperature range, the TlnT temperature curvature of the CTAT base-emitter voltage must also be corrected for.
Various attempts have been made to correct for the TlnT non-linearity of the CTAT voltage of the base-emitter of a transistor. U.S. Pat. No. 5,352,973 of Audy discloses a bandgap voltage reference circuit where the TlnT temperature curvature is corrected for. The bandgap voltage reference circuit of Audy comprises a Brokaw bandgap voltage reference cell and a correction cell. The Brokaw cell comprises first and second bipolar transistors which are arranged to develop a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The PTAT voltage difference is developed across a first resistor. The first and second transistors are operated with PTAT collector currents, and the collectors of the two transistors are held at a common voltage by an operational amplifier.
The correcting cell corrects for the TlnT curvature term, and comprises a third bipolar transistor which co-operates with one of the second transistor of the bandgap cell for developing a voltage across a second resistor which is proportional to the difference in the base-emitter voltages of the third transistor and the second transistor of the Brokaw cell. An operational amplifier drives the emitter of the third transistor until its collector current is at a substantially constant temperature insensitive value. This, thus, causes the difference voltage developed across the second resistor to have a TlnT curvature which is complementary to the TlnT curvature of the base-emitter CTAT voltage. Currents which flow through the first resistor in the Brokaw cell and the second resistor in the correction cell are summed in a third resistor embedded in the Brokaw cell for developing a corresponding voltage with a TlnT curvature complementary to the CTAT base-emitter voltage. The voltage developed across the third resistor is summed with the CTAT base-emitter voltage of the second transistor of the bandgap cell to provide a temperature stable and TlnT curvature corrected voltage reference.
However, while the voltage reference developed by the bandgap circuit of Audy is TlnT curvature corrected, and is thus temperature stable within a relatively wide temperature range, unfortunately, the bandgap circuit of Audy does not lend itself to easy implementation in a CMOS process. Furthermore, Audy relies on the PTAT current through the first resistor and the current through the second resistor which has a TlnT curvature complementary to the CTAT base-emitter voltage for developing the PTAT voltage with TlnT curvature across the third resistor.
U.S. Pat. No. 5,424,628 of Nguyen discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of bipolar transistors arranged in similar fashion to that of Audy in U.S. Pat. No. 5,352,973 for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors, which is then summed with a CTAT base-emitter voltage of one of the transistors of the bandgap cell. The Nguyen bandgap voltage reference circuit includes additional circuitry for providing a correction current signal, which is generated by a current squaring circuit, and is injected into the collector of one of the two transistors of the bandgap cell such that the collectors of the two transistors have unequal current values. The correction current is injected into the transistor which is to provide the CTAT base-emitter voltage of the voltage reference, and it is alleged that the collector current difference between the two transistors enables the elimination of the TlnT curvature of the CTAT base-emitter voltage. However, the circuitry required for implementing the bandgap voltage reference circuit of Nguyen is relatively complex, and additionally, it does not lend itself to a CMOS process.
U.S. Pat. No. 6,157,245 of Rincon-Mora discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of transistors arranged to develop a PTAT voltage proportional to the difference of the base-emitter voltages of the transistors, and this voltage is used to generate a PTAT current which is applied to one resistor of a resistor divider circuit comprising two resistors, across which the voltage reference is developed. The bandgap voltage reference circuit of Rincon-Mora also comprises a compensating circuit which generates a logarithmic operating temperature dependent current which is applied to the second resistor of the voltage divider network for developing a logarithmic temperature dependent correcting voltage across the second resistor. The voltages across the first and second resistors are summed to provide a voltage reference, which is allegedly temperature stable and TlnT curvature corrected. The circuitry of the Rincon-Mora bandgap voltage reference circuit is relatively complex, and does not easily lend itself to implementation in a CMOS process.
U.S. Pat. No. 5,512,817 of Nagaraj discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of bipolar transistors arranged for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The PTAT difference voltage is developed across a first resistor, and the developed PTAT difference voltage on the first resistor is scaled onto a second resistor through a current mirror circuit. The scaled voltage on the second resistor is summed with the CTAT base-emitter voltage of one of the transistors of the bandgap cell for providing the bandgap voltage reference. The voltage reference produced by this bandgap voltage reference circuit of Nagaraj does not contain any TlnT curvature correction.
U.S. Pat. No. 5,325,045 of Sundby discloses a bandgap voltage reference circuit which comprises a bandgap cell in which two stacks of bipolar transistors are arranged for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the transistors of the respective stacks. The PTAT voltage difference is developed across one of three resistors of a resistor divider network. The three resistors of the resistor divider network are negative temperature coefficient resistors, and voltages developed across the other two resistors of the resistor divider network are summed with the PTAT voltage. The voltages developed across all three resistors are summed with a CTAT base-emitter voltage of a separate bipolar transistor for producing the temperature curvature corrected voltage reference. In the circuit of Sundby, the TlnT temperature curvature correction is achieved by the use of the negative temperature coefficient resistors. However, the TlnT temperature curvature compensation of the bandgap voltage reference circuit of Sundby is not particularly accurate, and use of resistors with high temperature coefficients is not desirable.
U.S. Pat. No. 5,053,640 of Yum discloses a voltage reference circuit which comprises a bandgap cell for establishing a voltage reference, and a compensation circuit for compensating for non-linear temperature dependence of the bandgap voltage reference. The bandgap cell comprises two transistors arranged for developing a correcting PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The correcting PTAT voltage is developed across one resistor of a resistor divider network, and is summed with a compensating voltage developed across a compensation resistor in the resistor divider network. The compensation circuit comprises a switching circuit for switching a current through the compensation resistor which is varied in response to predetermined temperature threshold values for compensating for temperature curvature. However, since the compensating circuit varies the current flowing through the compensating resistor in steps in response to predetermined temperature threshold values, the temperature curvature correction provided by this circuit is relatively inaccurate, and furthermore, the circuit is a relatively complex circuit.
U.S. Pat. No. 4,939,442 of Carvajal discloses a bandgap voltage reference circuit which comprises a bandgap cell for developing a PTAT voltage proportional to the difference in base-emitter voltages of two bipolar transistors of the bandgap cell. The PTAT difference voltage is summed with CTAT base-emitter voltages of separate transistors for providing the voltage reference. However, in addition the PTAT voltage difference and the CTAT voltages of the two transistors are summed with voltages developed across two compensating resistors for compensating for the temperature curvature of the CTAT base-emitter voltages. One of the compensating resistors receives a compensating current for compensating at high temperatures, while the other compensating resistor receives a compensating current for compensating at low temperatures. A circuit for generating the high and low temperature currents is provided. However, the temperature curvature correction provided by the curvature correction circuit is of limited accuracy and does not adequately compensate for TlnT curvature. Furthermore, the circuit of Carvajal does not lend itself easily to implementation by a CMOS process.
U.S. Pat. No. 4,603,291 of Nelson discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of bipolar transistors which are arranged for developing a PTAT voltage proportional to the difference in base-emitter voltages of the two transistors across a first resistor. A correction circuit generates a correction current of the form TlnT which is added to the collector of one of the transistors of the bandgap cell for eliminating the TlnT curvature from the voltage reference of the band gap cell. However, the circuitry of Nelson is relatively complex, and does not lend itself to easy implementation in a CMOS process.
U.S. Pat. No. 6,218,822 of MacQuigg discloses a bandgap voltage reference circuit which includes a bandgap cell comprising a pair of bipolar transistors arranged to develop a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The PTAT voltage is summed with the CTAT base-emitter voltage of one of the transistors to produce the reference voltage. Non-linear resistors, such as n-type lightly doped drain diffusion resistors, which have a curvature characteristic opposite to that of the voltage reference of the bandgap cell are provided for correcting the temperature curvature of the voltage reference. Provision is made for trimming the non-linear resistors. The temperature stability of the voltage reference of this circuit is limited, since the curvature correction is reliant solely on non-linear resistors.
U.S. Pat. No. 4,808,908 of Lewis discloses a bandgap voltage reference circuit which comprises a bandgap cell comprising a pair of bipolar transistors arranged for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the two transistors. The PTAT difference voltage is summed with a CTAT base-emitter voltage of a transistor to produce the voltage reference. A compensating voltage is developed across compensating resistors is summed with the CTAT base-emitter voltage and the PTAT difference voltage for correcting for first and second derivatives of the bandgap cell output as a function of temperature. This circuit of Lewis does not easily lend itself to implementation in a CMOS process, and additionally, TlnT temperature curvature correction is limited.
There is therefore a need for a bandgap voltage reference circuit which overcomes the problems of known bandgap voltage reference circuits, and which preferably lends itself readily to implementation in a CMOS process, and provides a relatively temperature stable voltage reference which is corrected for TlnT curvature over a reasonable temperature range. There is also a need for a PTAT voltage generating circuit for generating a PTAT voltage which is complementary to a CTAT base-emitter transistor voltage, and which preferably readily lends itself to implementation in a CMOS process.
The present invention is directed towards providing such a bandgap voltage reference circuit and a PTAT voltage generating circuit, and the invention is also directed towards a method for generating such a PTAT voltage and a bandgap voltage reference.
According to the invention there is provided a bandgap voltage reference circuit for providing a temperature stable voltage reference with TlnT temperature curvature correction, the bandgap voltage reference circuit comprising at least one first transistor and at least one second transistor supplied with respective PTAT currents, the at least one second transistor being operable at a current density lower than the current density at which the at least one first transistor is operable, and co-operating with the at least one first transistor for developing a correcting PTAT voltage proportional to the difference in the base-emitter voltages of the first and second transistors for combining with an uncorrected transistor base-emitter CTAT voltage for producing the voltage reference, wherein a CTAT correcting current is supplied to one of the at least one second transistors along with the PTAT current for developing the correcting PTAT voltage with a curvature complementary to the TlnT temperature curvature of the uncorrected transistor base-emitter CTAT voltage, so that when the correcting PTAT voltage is combined with the uncorrected transistor base-emitter CTAT voltage, the voltage reference produced is temperature stable and TlnT temperature curvature corrected.
In one embodiment of the invention the ratio of the CTAT correcting current to the PTAT current is selected in response to the ratio of the area of the at least one second transistor to the area of the at least one first transistor.
Preferably, a primary resistor is provided co-operating with the first and second transistors so that the correcting PTAT voltage corresponding to the difference in the base-emitter voltages of the first and second transistors is developed across the primary resistor.
In one embodiment of the invention the at least one first transistor is connected between a first voltage level and a second voltage level, the second voltage level being different to the first voltage level, and the at least one second transistor is connected in series with the primary resistor between the first voltage level and the second voltage level.
Preferably, the PTAT current which is supplied to the second transistor to which the primary resistor is connected is supplied through the primary resistor to the second transistor.
In one embodiment of the invention the collectors of the first and second transistors are held at a common voltage level, and the PTAT currents are supplied to the emitters of the first and second transistors, the CTAT correcting current being supplied to the emitter of the second transistor, and preferably, the common voltage level is the same as the second voltage level.
In one embodiment of the invention the primary resistor is connected between the first voltage level and the emitter of one of the at least one second transistors.
In another embodiment of the invention a secondary resistor is provided, and the correcting PTAT voltage is reflected from the primary resistor across the secondary resistor, the secondary resistor co-operating with the transistor, the uncorrected base-emitter CTAT voltage of which is to be combined with the correcting PTAT voltage for summing the correcting PTAT voltage with the uncorrected base-emitter CTAT voltage of the transistor for producing the voltage reference.
Preferably, the correcting PTAT voltage is scaled from the primary resistor to the secondary resistor.
In one embodiment of the invention the transistor the uncorrected base-emitter CTAT voltage of which is to be combined with the PTAT correcting voltage is one of the at least one first transistor.
In another embodiment of the invention the CTAT correcting current is selected in response to the gain of the correcting PTAT voltage from the primary resistor to the secondary resistor.
In one embodiment of the invention the circuit comprises one first transistor and one second transistor, the bases of the first and second transistors being held at the second voltage level.
Alternatively, a plurality of first transistors are provided arranged in a first transistor stack, so that the base-emitter voltages of the first transistors are summed to provide a base-emitter voltage of the first stack, and a plurality of second transistors are arranged in a second transistor stack so that the sum of the base-emitter voltages of the second transistors are summed to provide a base-emitter voltage of the second stack, the number of second transistors in the second stack corresponding to the number of first transistors in the first stack, the first and second transistors being supplied with respective PTAT currents.
In one embodiment of the invention the base of each first transistor is connected to the emitter of the next lower first transistor in the first transistor stack, and the base of each second transistor is connected to the emitter of the next lower second transistor in the second transistor stack.
In another embodiment of the invention the primary resistor is connected between the topmost second transistor in the second transistor stack and the first voltage level.
In a further embodiment of the invention the CTAT correcting current is supplied to the lowermost second transistor of the second transistor stack.
In another embodiment of the invention the bases of the lowermost first and second transistors of the respective first and second transistor stacks are connected to the second voltage level.
In a further embodiment of the invention the transistor the uncorrected base-emitter CTAT voltage of which is to be combined with the correcting PTAT voltage is the lowermost first transistor of the first transistor stack.
Preferably, the CTAT correcting current is derived from the uncorrected base-emitter CTAT voltage of the transistor with which the correcting PTAT voltage is combined.
In one embodiment of the invention a first calibration circuit is provided for adjusting the CTAT correcting current.
In another embodiment of the invention a second calibration circuit is provided for adjusting the PTAT current supplied through the secondary resistor for adjusting the correcting PTAT voltage developed across the secondary resistor.
In a further embodiment of the invention the second calibration circuit provides for adjusting the PTAT current supplied to the transistor, the uncorrected base-emitter CTAT voltage of which is to be combined with the correcting PTAT voltage.
In one embodiment of the invention the circuit is implemented in CMOS.
Additionally, the invention provides a PTAT voltage generating circuit for generating a PTAT voltage with a curvature complementary to an uncorrected TlnT temperature curvature of a base-emitter CTAT voltage of a transistor, the PTAT voltage generating circuit comprising at least one first transistor and at least one second transistor supplied with respective PTAT currents, the at least one second transistor being operable at a current density lower than the current density at which the at least one first transistor is operable, and co-operating with the at least one first transistor for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the first and second transistors, wherein a CTAT correcting current is supplied to one of the at least one second transistors along with the PTAT current for developing the PTAT voltage with the curvature complementary to the TlnT temperature curvature of an uncorrected transistor base-emitter CTAT voltage.
In one embodiment of the invention the ratio of the CTAT current to the PTAT current is selected in response to the ratio of the area of the at least one second transistor to the area of the at least one first transistor.
Preferably, a primary resistor is provided co-operating with the first and second transistors so that the PTAT voltage corresponding to the difference in the base-emitter voltages of the first and second transistors is developed across the primary resistor.
In one embodiment of the invention the at least one first transistor is connected between a first voltage level and a second voltage level, the second voltage level being different to the first voltage level, and the at least one second transistor is connected in series with the primary resistor between the first voltage level and the second voltage level.
Preferably, the PTAT current which is supplied to the second transistor to which the primary resistor is connected is supplied through the primary resistor to the second transistor.
Advantageously, the collectors of the first and second transistors are held at a common voltage level, and the PTAT currents are supplied to the emitters of the first and second transistors, the CTAT correcting current being supplied to the emitter of the second transistor. Preferably, the common voltage level is the same as the second voltage level.
In one embodiment of the invention a plurality of first transistors are provided arranged in a first transistor stack, the base of each first transistor being connected to the emitter of the next lower first transistor in the first transistor stack, so that the base-emitter voltages of the first transistors are summed to provide a base-emitter voltage of the first stack, and a plurality of second transistors arranged in a second transistor stack, the base of each second transistor being connected to the emitter of the next lower second transistor in the second transistor stack, so that the sum of the base-emitter voltages of the second transistors are summed to provide a base-emitter voltage of the second stack, the number of second transistors in the second stack corresponding to the number of first transistors in the first stack, the first and second transistors being supplied with respective PTAT currents.
In another embodiment of the invention the primary resistor is connected between the topmost second transistor in the second transistor stack and the first voltage level, and the CTAT correcting current is supplied to the lowermost second transistor of the second transistor stack, the bases of the lowermost first and second transistors of the respective first and second transistor stacks being connected to the second voltage level.
Further the invention provides a method for generating a temperature stable bandgap voltage reference with TlnT temperature curvature correction, the method comprising the steps of:
providing at least one first transistor and at least one second transistor co-operating with the at least one first transistor for developing a correcting PTAT voltage proportional to the difference in the base-emitter voltages of the first and second transistors,
supplying the at least one first transistor and the at least one second transistor with respective PTAT currents,
operating the at least one second transistor at a current density lower than the current density at which the at least one first transistor is being operated for developing the correcting PTAT voltage, and
combining the correcting PTAT voltage with an uncorrected transistor base-emitter CTAT voltage for producing the voltage reference, wherein the method comprises the further step of
supplying a CTAT correcting current to one of the at least one second transistors along with the PTAT current for developing the correcting PTAT voltage with a curvature complementary to the TlnT temperature curvature of the uncorrected transistor base-emitter CTAT voltage, so that when the correcting PTAT voltage is combined with the uncorrected transistor base-emitter CTAT voltage, the voltage reference produced is temperature stable and TlnT temperature curvature corrected.
In one embodiment of the invention the PTAT currents are supplied to the emitters of the first and second transistors and the CTAT correcting current is supplied to the emitter of the second transistor.
In another embodiment of the invention the ratio of the CTAT correcting current to the PTAT current is selected in response to the ratio of the area of the at least one first transistor to the area of the at least one second transistor.
The invention also provides a method for generating a PTAT voltage with a curvature complementary to an uncorrected TlnT temperature curvature of a base-emitter CTAT voltage of a transistor, the method comprising the steps of:
providing at least one first transistor and at least one second transistor co-operating with the at least one first transistor for developing a PTAT voltage proportional to the difference in the base-emitter voltages of the first and second transistors,
supplying the at least one first transistor and the at least one second transistor with respective PTAT currents, and
operating the at least one second transistor at a current density lower than the current density at which the at least one first transistor is being operated for developing the PTAT voltage proportional to the difference in the base-emitter voltages of the first and second transistors, wherein the method comprises the further step of
supplying a CTAT correcting current to one of the at least one second transistors along with the PTAT current for developing the PTAT voltage with a curvature complementary to the TlnT temperature curvature of an uncorrected transistor base-emitter CTAT voltage.
In one embodiment of the invention the PTAT currents are supplied to the emitters of the first and second transistors, and the CTAT correcting current is supplied to the emitter of the second transistor.
In another embodiment of the invention the ratio of the CTAT correcting current to the PTAT current is selected in response to the ratio of the area of the at least one first transistor to the area of the at least one second transistor.
The advantages of the invention are many. The bandgap voltage reference provides a temperature stable voltage reference which is corrected for TlnT temperature curvature, and the voltage reference is stable over a relatively wide temperature range, and in particular over the temperature range of xe2x88x9240xc2x0 C. to +120xc2x0 C. Indeed, it is believed that the voltage reference is temperature stable over an even wider temperature range. Furthermore, the bandgap voltage reference circuit according to the invention is a relatively non-complex circuit, and can be readily easily implemented in a CMOS process with a relatively low die area requirement. This advantage has been achieved by virtue of the fact that the circuit can be constructed with the collectors of the first and second transistors tied to the same voltage level, which can be ground or any other suitable common voltage level. The PTAT voltage generated by the bandgap voltage reference circuit according to the invention as well as having a positive temperature coefficient, also has a curvature of TlnT form which is complementary to the TlnT curvature of a CTAT base-emitter voltage of a transistor, and thus, the PTAT voltage developed by the bandgap voltage reference circuit is ideally suited to correcting for the TlnT temperature curvature of the negative temperature coefficient of the base-emitter CTAT voltage of a transistor for producing a temperature stable TlnT temperature curvature corrected reference voltage. The fact that the CTAT correcting current is derived from the base-emitter CTAT voltage of one of the first transistors leads to the simplicity and temperature stability of the circuit.
The simplicity of the bandgap voltage reference circuit and the temperature stability of the voltage reference are largely achieved by virtue of the fact that correction for the transistor base-emitter CTAT voltage and the TlnT temperature curvature component of the transistor base-emitter CTAT voltage are corrected for in the same bandgap cell. In other words, both the correcting PTAT voltage and the TlnT curvature component which is complementary to the transistor base-emitter TlnT temperature curvature component are developed in the same bandgap cell. Both components of the correction voltage, in other words, the correcting PTAT voltage and the complementary TlnT temperature curvature correction are developed in the bandgap cell and are developed across the primary resistor in the bandgap cell. The correcting PTAT voltage with the complementary TlnT temperature curvature correction which are simultaneously developed across the primary resistor can then be readily reflected and if desired scaled onto the secondary resistor for summing with the uncorrected transistor base-emitter CTAT voltage.
In particular, the simplicity of the circuit according to the invention is achieved by virtue of the fact that the correcting PTAT voltage with the TlnT temperature curvature correction voltage are developed simultaneously across one single resistor, namely, the primary resistor in the bandgap cell. This leads to considerable simplification of the bandgap circuit, and furthermore, minimises the sensitivity of the bandgap circuit to process variations.
A further advantage of the invention relates to the ease with which the bandgap voltage circuit may be trimmed during calibration. Since the TlnT curvature component of the correcting PTAT voltage is developed across the primary resistor, along with the PTAT voltage, trimming of the TlnT temperature curvature component can readily easily be achieved by trimming the proportion of the CTAT correcting current which is summed with the PTAT current and supplied to the emitter of the second transistor. In other words, trimming of the TlnT curvature component is carried out by varying the ratio of the CTAT correcting current to the PTAT current supplied to the second transistor until the desired TlnT curvature component is achieved. Thus, a first calibration circuit for trimming the CTAT correcting current can readily easily be provided as a simple current DAC. This method of trimming the TlnT temperature curvature component is significantly less complex than trimming methods required in prior art bandgap voltage reference circuits. In general, in prior art bandgap voltage reference circuits, trimming of the TlnT temperature curvature requires trimming the resistors across which the TlnT temperature curvature component is developed. This requires providing a resistor network across which the TlnT temperature curvature correction voltage is developed, and provision is required for selectively switching the resistors of the resistor network into and out of the resistor network until the TlnT temperature curvature correcting voltage has been properly corrected.
The invention and its advantages will be more clearly understood from the following description of some preferred embodiments thereof, which are given by way of example only, with reference to the accompanying drawings.